Polarization Resistant Solar Cell Design Using SiCN

ABSTRACT

A polarization resistant solar cell is provided. The solar cell uses a dual layer dielectric stack disposed on the front surface of the cell. The dielectric stack consists of a passivation layer disposed directly on the front cell surface and comprised of either SiO x  or SiON, and an outer AR coating comprised of SiCN.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of the filing date of U.S.Provisional Patent Application Ser. No. 61/279,842, filed Oct. 27, 2009,the disclosure of which is incorporated herein by reference for any andall purposes.

FIELD OF THE INVENTION

The present invention relates generally to solar cells and, inparticular, to a polarization resistant solar cell design.

BACKGROUND OF THE INVENTION

Photovoltaic cells, commonly referred to as solar cells, are well knownsemiconductor devices that convert photons into electrical energy. FIG.1 provides a cross-sectional view of a conventional solar cell 100 thatincludes a substrate 101 of a first conductivity type, the substratefrequently comprised of silicon, and a layer 103 of a secondconductivity type formed on the substrate, thereby forming a p-njunction at the interface. Solar cell 100 also includes a rear surfaceelectrode 105 that is in contact with at least a portion of substrate101, and a front surface electrode 107 that is in contact with at leasta portion of layer 103. When light falls on solar cell 100,electron-hole pairs are created, and are converted by the solar cellinto electrical energy.

To enhance the performance of a conventional solar cell, typically adielectric layer 109 is deposited on the front surface of the solarcell. Dielectric layer 109 serves dual purposes. First, it acts as ananti-reflection (AR) coating, thereby increasing the percentage ofincident light that passes into cell 100, resulting in improvedconversion efficiency. Second, it forms a passivation layer on thesurface of layer 103. In some solar cells, dielectric layer 109 iscomprised of a pair of layers; an inner passivation layer and an outerAR layer.

Solar cells are becoming commonplace in a wide range of applications,both due to the increase in energy costs and the growing environmentalconcerns associated with traditional energy sources. The switch to solarenergy has been aided by the gradually improving performance of solarcells and the steady decrease in cell cost. In a typical application,for example a solar array for use on a residential or commercialroof-top or in a solar farm, a large number of solar panels areelectrically connected together, each solar panel comprised of a largearray of solar cells.

When a solar panel or an array of solar panels is put into operation, ahigh voltage in excess of 100V may exist between the panel frame orexternal grounding and one or more terminals of the individual devices.As a result, an electric field is generated that may create a charge onthe dielectric layer or layers used in the fabrication of the cell, forexample, passivation and AR layer 109 of FIG. 1. Over time, theaccumulation of charge on the dielectric layer(s) leads to surfacepolarization which, in turn, induces an electric field on the cell's p-njunction. As a result, shunt resistance and p-n junction characteristicsare significantly degraded, leading to a major reduction in cellconversion efficiency and potentially complete cessation of cell poweroutput. Accordingly, what is needed is a solar cell design that isresistant to surface polarization but does not significantly affect thefabrication process, the overall cell manufacturing cost, or the cell'sperformance. The present invention provides such a design.

SUMMARY OF THE INVENTION

The present invention provides a solar cell that is resistant to thepolarization effect, the solar cell using a dual layer dielectric stackdisposed on the front surface of the cell. The dielectric stack consistsof a passivation layer disposed directly on the front cell surface andcomprised of either SiO_(x) or SiON, and an outer AR coating comprisedof SiCN.

A further understanding of the nature and advantages of the presentinvention may be realized by reference to the remaining portions of thespecification and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a conventional silicon solar cell;and

FIG. 2 provides a cross-sectional view of an exemplary device structurein accordance with the invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 2 provides a cross-sectional view of a preferred solar cell devicestructure 200 in accordance with the invention. Silicon substrate 201may be of either p- or n-type. As with a conventional solar cell, asilicon layer 203 of a second conductivity type is formed on substrate201, thereby forming the cell's p-n junction. A rear surface electrode205, for example comprised of aluminum, contacts at least a portion ofsubstrate 201 or, as shown, the entire back surface of the substrate. Tocontact the front surface of the device, more specifically layer 203,preferably a plurality of front surface electrodes 207, preferablycomprised of silver, are applied to the device's front surface, forexample using a finger/busbar configuration as is well known by those ofskill in the art.

In accordance with the invention, a two layer dielectric stack isapplied to the front surface of cell 200. The dielectric stack iscomprised of an inner passivation layer 209 applied directly to layer203, and an outermost AR layer 211. Passivation layer 209 may befabricated from either a silicon oxide, i.e., SiO_(x), or siliconoxynitride, i.e., SiON. AR coating layer 211 is fabricated fromamorphous silicon carbon nitride (SiCN). The inventors have found thatthe use of these two dielectric layers substantially reduces, if notaltogether eliminates, the polarization effect typically experienced bythe solar cells contained within a module.

In order to achieve the desired level of surface passivation, thethickness of passivation layer 209 is in the range of 1 to 100nanometers, preferably in the range of 1 to 50 nanometers, and morepreferably in the range of 2 to 30 nanometers. If layer 209 is comprisedof SiON, rather than SiO_(x), then the amount of oxygen and nitrogen inthe layer is defined by the fraction of oxygen within the layer, i.e.,the ratio between oxygen and the sum of oxygen and nitrogen (i.e.,O/(O+N)). Preferably the fraction of oxygen is in the range of 0.01 to0.99, more preferably in the range of 0.1 to 0.9, and still morepreferably in the range of 0.4 to 0.9.

AR layer 211, comprised of SiCN as previously noted, has a thickness inthe range of 1 to 200 nanometers, preferably in the range of 20 to 120nanometers, and more preferably in the range of 40 to 100 nanometers.The combined thickness of layers 209 and 211 is in the range of 2 to 300nanometers with a refractive index in the range of 1.5 to 2.4. In atleast one embodiment, SiCN layer 211 is hydrogenated.

It will be appreciated that any of a variety of techniques may be usedto form layer 203, form dielectric layers 209 and 211, and applycontacts 205 and 207, and that the present design is not limited to aspecific fabrication methodology. In an exemplary process in which layer209 is comprised of SiO_(x) layer 209 is formed using thermal oxidation,chemical oxidation or CVD oxide deposition. In an exemplary process inwhich layer 209 is comprised of SiON, layer 209 is deposited using anin-situ silicon oxynitride deposition process (e.g., CVD deposition ofSiON). In an alternate process, the SiON layer is formed by firstdepositing an oxide layer, preferably greater than 4 nanometers inthickness, on top of silicon layer 203, for example using thermaloxidation, chemical oxidation or CVD oxide deposition. Next, a nitridelayer is deposited in such a way that the silicon oxide transforms intosilicon oxynitride of the desired thickness and composition.Alternately, the previously grown oxide layer can be annealed in anitrogen environment, thereby transforming the silicon oxide to thedesired silicon oxynitride.

It should be understood that identical element symbols used on multiplefigures refer to the same structure, or structures of equalfunctionality. Additionally, the accompanying figures are only meant toillustrate, not limit, the scope of the invention and should not beconsidered to be to scale.

As will be understood by those familiar with the art, the presentinvention may be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. Accordingly, thedisclosures and descriptions herein are intended to be illustrative, butnot limiting, of the scope of the invention.

1. A solar cell comprising: a substrate comprised of silicon of a firstconductivity type; a layer of silicon of a second conductivity typedisposed on said substrate; a passivation layer disposed on said layerof silicon, wherein said passivation layer is selected from the groupconsisting of silicon oxide and silicon oxynitride; and ananti-reflection (AR) layer disposed on said passivation layer, whereinsaid AR layer is comprised of silicon carbon nitride (SiCN).
 2. Thesolar cell of claim 1, wherein said passivation layer has a thickness ofbetween 1 and 100 nanometers.
 3. The solar cell of claim 1, wherein saidpassivation layer has a thickness of between 1 and 50 nanometers.
 4. Thesolar cell of claim 1, wherein said passivation layer has a thickness ofbetween 2 and 30 nanometers.
 5. The solar cell of claim 1, wherein saidAR layer has a thickness of between 1 and 200 nanometers.
 6. The solarcell of claim 1, wherein said AR layer has a thickness of between 20 and120 nanometers.
 7. The solar cell of claim 1, wherein said AR layer hasa thickness of between 40 and 100 nanometers.
 8. The solar cell of claim1, wherein a dielectric stack comprised of said passivation layer andsaid AR layer has a refractive index in the range of 1.5 to 2.4.
 9. Thesolar cell of claim 1, wherein said passivation layer is comprised ofsaid SiON, and wherein a ratio of oxygen within said layer of SiON tothe sum of oxygen and nitrogen within said layer of SiON is in the rangeof 0.01 to 0.99.
 10. The solar cell of claim 1, wherein said passivationlayer is comprised of said SiON, and wherein a ratio of oxygen withinsaid layer of SiON to the sum of oxygen and nitrogen within said layerof SiON is in the range of 0.1 to 0.9.
 11. The solar cell of claim 1,wherein said passivation layer is comprised of said SiON, and wherein aratio of oxygen within said layer of SiON to the sum of oxygen andnitrogen within said layer of SiON is in the range of 0.4 to 0.9. 12.The solar cell of claim 1, wherein said AR layer is hydrogenated. 13.The solar cell of claim 1, further comprising a first metal electrodeformed on a back surface of said substrate and a second metal electrodein contact with said silicon layer of said second conductivity type.